GM put their 2015 Chevrolet Silverado 2500 against a 2015 Ford F-250 over an exaggerated "bumpy road" ramp test forcing both trucks to flex between the cab and the bed. The Chevy stayed a lot straighter.
Former NFL player slash actor Howie Long was on hand to add some witty commentary like saying; "I'm confused... some trucks claim to own work, but if the back of your truck doesn't open up, how are you gonna get any work done?"
A pretty obvious dig at Ford Super Duty's tagline; "We own work," in regards to the tailgate being stuck shut when the truck was parked on an awkward angle. The Silverado exhibits higher torsional rigidity over the same obstacle, flexing significantly less and still being able to open its tailgate.
Having driven both of the trucks featured here, I can verify the Ford F-250 exhibits a lot more visible bending between the bed and cab over uneven terrain. That said, you have to get the trucks over some pretty wonky washboards to notice.
GM's schtick here isn't a new one... here's almost the same video from three years prior. Hell, they've even got Howie saying some of the same lines!
Basically, the flex is bad because you want you suspension to be taking the stress of a shifting surface... not the frame that's holding the whole truck together.